Yvon Le Page

CRYSTMET: a database of the structures and powder patterns of metals and intermetallics

CRYSTMET is a database of critically evaluated crystallographic data for metals (including alloys, intermetallics and minerals) and associated bibliographic, chemical and physical information. Also included are simulated powder diffraction patterns for all of the entries. The database currently contains almost 70,000 entries and covers the literature exhaustively from 1922 to the present. The database is available on CD-ROM with search/analysis software for use on personal computers. This software can be used with any database in the appropriate format; currently CRYSTMET and the ICSD databases are available. This paper describes the database content, the procedures used in its construction, the software made available to the user and a number of potential uses for the data.

The crystal and molecular structure of bis(biscyclopentadienylchlorotitanium) oxide

Abstract Bis(biscyclopentadienylchlorotitanium) oxide, Cp 2 TiCl) 2 O, crystallizes in the enantiomorphic space groups P 3 1 21 and P 3 2 21 with a 7.742(1), c 27.177(7) A. In this study only twins were obtained. On the basis of intensity data for 1106 independent reflections, the structure is described in P 3 1 21 with final residuals of R F = 3.27% and R WF = 2.53%. The molecule is very similar to that of the analogous zirconium compound. Mean bond lengths are CpTi(π), 2.09; ClTi, 2.41; and TiO, 1.84 A. The TiOTi bond angle is 173.8°.

Quantum software interfaced with crystal-structure databases: tools, results and perspectives

Version 2.0 of Toths Materials Toolkit runs under Windows and prepares ASCII input files for popular ab initio packages such as ABINIT, VASP etc. Those packages, obtainable from their respective developers, may run in desktop or supercomputer setups with Linux or Windows operating systems. The Toolkit input is taken at will from a direct plug into CRYSTMET, with 93000 crystal-structure entries for metals and inorganic compounds, from CIF files of public-domain crystal-structure databases, or cut-and-paste from electronic journals followed by minimal free-format editing. The collection of fully general and highly graphical tools grouped on two command screens operates on the structure description stored in an editable ASCII screen. After the model has been searched, modified and evaluated in a few keystrokes with the above tools, its ASCII input files for a selection of ab initio packages are produced by selecting the meaningful flags and run options on a dialog. The tedious structure manipulation or decomposition into multiple simulations is performed in the background. Execution is followed by production of a plain-English job report. Four examples among the numerous possible applications of the Toolkit illustrate the fact that daunting topics, like the symmetry of chlorapatite, the voids and channels in the hydrogen-storage material EuNi5, the energy per unit area of the contact plane for spinel twin in diamond, and the hardness of lonsdaleite versus diamond, are amenable to processing by materials scientists more versed in experiment than theory. The manual with tutorials and availability information can be found at http://www.tothcanada.com/toolkit/.

Geometrical parameterization of the crystal chemistry of P63/m apatites : comparison with experimental data and ab initio results

Experimental structure refinements and ab initio simulation results for 18 published, fully ordered P6(3)/m (A;{\rm I}_4)(A;{\rm II}_6)(BO4)6X2 apatite end-member compositions have been analyzed in terms of a geometric crystal-chemical model that allows the prediction of unit-cell parameters (a and c) and all atom coordinates. To an accuracy of +/- 0.025 A, the magnitude of c was reproduced from crystal-chemical parameters characterizing chains of ...-A(II)-O3-B-O3-A(II)-... atoms, whereas that of a was determined from those describing (A(I)O6)-(BO4) polyhedral arrangements. The c/a ratio could be predicted to +/-0.2% using multi-variable functions based on geometric crystal-chemical model predictions, but could not be ascribed to the adjustment of a single crystal-chemical parameter. The correlations observed between algebraically independent crystal-chemical parameters representing the main observed polyhedral distortions reveal them as the minimum-energy solution to accommodate misfit components within this flexible structure type. For materials with given composition, good agreement (within +/- 0.5-2.0%) of ab initio crystal-chemical parameters was observed with only those from single-crystal refinements with R </= 4.0%. Agreement with single-crystal work with R > 4.0% was not as good, while the scatter with those from Rietveld refinements was considerable. Accordingly, ab initio cell data, atomic coordinates and crystal-chemical parameters were reported here for the following compositions awaiting experimental work: (Zn,Hg)10(PO4)6(Cl,F)2, (Ca,Cd)10(VO4)6Cl2 and (Ca,Pb,Cd)10(CrO4)6Cl2.

Geometrical parameterization of the crystal chemistry of P63/m apatite. II. Precision, accuracy and numerical stability of the crystal-chemical Rietveld refinement

A script developed for crystal-chemical Rietveld refinement of P63/m apatite with TOPAS is implemented in parallel with standard structure refinement. Least-squares standard uncertainty (s.u.) values for directly extracted crystalchemical parameters are nearly an order of magnitude lower than those obtained indirectly by analysis of atom coordinates derived by standard Rietveld refinement. This amazing finding originates partly in the reduction of the number of refinement parameters from 21 to 17 and partly in the fact that cell data now derive from crystal-chemical parameters instead of vice versa. Great precision and accuracy otherwise funneled into unit-cell parameters is then more distributed among mostly crystal-chemical distance parameters. The least-squares s.u. values are supported by analysis of numerous refinements of the same experimental data with added artificial intensity noise. Structural parameters from single-crystal results agree better with those extracted by crystal-chemical refinement. On the basis of singular value decomposition analyses performed using the program SVDdiagnostic [Mercier et al. (2006). J. Appl. Cryst. 39, 458‐465], crystal-chemical and standard Rietveld refinements are shown to have similar numerical stability. Crystal-chemical parameters extracted by direct Rietveld refinement, therefore, are more precise than, more accurate than and numerically as reliable as those derived from analysis of regular crystallographic refinement of the same data.

Crystal growth of Ti n O2n-1 oxides(n=2 to 9)

Crystals of titanium oxides, TinO2n-1 with n=2 to 9, were grown by chemical vapour transport. Chlorine was found to be the best transporting agent. Apart from Ti2O3, for which crystals up to 7 mm long were obtained in 6 days, all phases were obtained in conditions leading to very low growth rates and a shift in composition between the charge and growth zones. It is shown that this shift is governed by the equilibrium oxygen pressure in the transport tubes and can be accurately predicted. Most of the millimetresize samples obtained were twinned.

Ab initio constrained crystal-chemical Rietveld refinement of Ca10(VxP1 − xO4)6F2 apatites

Extraction of reliable bond distances and angles for Ca10(VxP1-xO4)6F2 apatites using standard Rietveld refinement with Cu Kalpha X-ray powder data was significantly impaired by large imprecision for the O-atom coordinates. An initial attempt to apply crystal-chemical Rietveld refinements to the same compounds was partly successful, and exposed the problematic determination of two oxygen-metal-oxygen angles. Ab initio modeling with VASP in space groups P6(3)/m, P2(1)/m and Pm showed that both these angular parameters exhibited a linear dependence with the vanadium content. Stable crystal-chemical Rietveld refinements in agreement with quantum results were obtained by fixing these angles at the values from ab initio simulations. Residuals were comparable with the less precise standard refinements. The larger vanadium ion is accommodated primarily by uniform expansion and rotation of BO4 tetrahedra combined with a rotation of the Ca-Ca-Ca triangular units. It is proposed that the reduction of symmetry for the vanadium end-member is necessary to avoid considerable departures from formal valences at the AII and B sites in P6(3)/m. The complementarity of quantum methods and structural analysis by powder diffraction in cases with problematic least-squares extraction of the crystal chemistry is discussed.

Growth of Ti9O17 crystals by chemical vapor transport

Abstract Crystals of Ti 9 O 17 up to 2 mm long were grown by chlorine vapor transport from a starting mixture of composition Ti 6 O 11 , using low transport rate conditions ( P Cl 2 = 30 Torr, temperature gradient 55°C). The shift in composition between the charge and growth zones (respectively at 1015 and 960°C) corresponds to a constant oxygen pressure in the transport tube. Smaller multiple crystals of Ti 6 O 11 and Ti 8 O 15 were also formed in the charge zone.

Rational ab initio modeling for low energy hydrogen-bonded phyllosilicate polytypes

In a series of recent papers implementing ab initio DFT modeling with VASP, we have explored the kaolin system at zero pressure, kaolinite under pressure up to 60 GPa and the known kaolin phases under moderate pressure at 10 GPa. We summarize here the concepts, conclusions and falsifiable predictions printed in this series of papers, stressing independent and recent experimental results. A new rationalization of the kaolin system results, clarifying its stability diagram, its diagenesis and its solid-state phase transformations. The existence at moderate pressure of two new translations −a/3 and (a + b)/3, not possible at zero or low pressure, leading to five-fold coordination for Si was correctly predicted. Two newly and independently observed kaolinite polytypes (kaolinite II and III) were also correctly predicted. The existence of a still unobserved (SU) kaolinite IV phase is predicted at a pressure not higher than 60 GPa. Finally, transformations of dickite II into SU dickite III and nacrite into SU nacrite II are predicted to occur around 10 GPa. Optimized crystal structures predicted by ab initio modeling for the most likely low enthalpy polytypes are printed, which should simplify their identification when they will be observed, as they did for kaolinite II and III. Concepts developed here for kaolin minerals are generally applicable to hydrogen-bonded phyllosilicate polytypes or other hydrogen-bonded layered systems. This series of papers then implements a new way of expanding knowledge about experimentally difficult systems: inexpensive and relatively fast quantum computations can bring support to experimental results when conflicting reports exist in the literature, as well as produce predictions that are easily falsifiable experimentally, thus pointing to fruitful directions for new experiments. This loop of quantum computation followed by critical experiments results in faster and cheaper scientific progress as seen here on the kaolin system.

SVDdiagnostic, a program to diagnose numerical conditioning of Rietveld refinements

Singular value decomposition (SVD) of the matrix of normal equations is used here both passively to assess numerical stability, and actively to troubleshoot problem refinements, singular or not. Such systems can then either be cured by rank reduction or solved with arbitrary-precision arithmetic carrying a number of digits known to be sufficient. SVD analysis provides objective information about such required rank reduction or number of digits. Pre-conditioning of the normal matrix is seen to decrease its condition number by many orders of magnitude in actual cases, illustrating its great practical usefulness. The methods and tools developed here have general applicability to diagnose problems with least squares, in particular ill-conditioned Rietveld refinements. Crystal-chemical and standard refinements described in the work by Mercier et al. [J. Appl. Cryst. (2006), 39. 369-375] are shown to have similar numerical stability. The program SVDdiagnostic is freely available at http://www.tothcanada.com.